The present invention relates to echo suppressors, and in particular to an improved single frequency (SF) signaling tone transparent echo suppressor for long distance telephone communication systems.
Long distance telephone communication facilities generally comprise four-wire transmission circuits between central switching offices in different local exchange areas, and two-wire transmission circuits within each exchange area connecting the individual subscribers therein with the switching office. A call between subscribers in different exchange areas is carried over a two-wire transmission circuit within each of the areas and a four-wire transmission circuit extending between the central switching offices of the areas, with conversion of speech energy between each two-wire circuit and the four-wire circuit, and vice versa, being effected by a hybrid circuit. Ideally, the hybrid circuit ports perfectly match the impedances of the two-wire and the four-wire circuits, and even more importantly the hybrid circuit balance network impedance perfectly matches the impedance of the two-wire circuit, so that signals transmitted from one exchange area to the other will not be reflected or returned to the one area as an "echo" signal. Unfortunately, due to impedance differences which inherently exist between two-wire as well as four-wire circuits, and because impedances must be matched at each frequency in the voice band, it is virtually impossible to provide a hybrid balance network which will ideally match the impedance of any particular two-wire transmission circuit. Echo is, therefore, characteristically part of a long distance telephone system.
Echo of the type described above is tolerable, so long as the time delay in the echo path is relatively short, shorter than, say, 40 or 45 milliseconds. To reduce echo signals in longer delay telephone circuits to a tolerable level, echo suppressors are used for selectively inserting attenuation or suppression loss into transmit and/or receive channels of four-wire transmission circuits characterized by appreciable propagation delay to suppress the echo signals. Without echo suppression a portion of a speaking subscriber's voice signal, carried by two of the four wires to a listening subscriber, is reflected back to him over the other two wires and is heard as an echo of his own voice. An echo signal which has a return delay of 40-45 milliseconds or longer is at the very least disturbing and possibly utterly confusing to an average speaker, and must be removed. Of course, echo delay times in excess of 45 milliseconds are not uncommon, and in modern satellite communications the delays often exceed 600 milliseconds.
Echo suppressors may be either of the "split" or "full" type. With split echo suppressors, a separate echo suppressor is used for each of a near end and a far end subscriber, with the near end echo suppressor protecting the far end subscriber against his own voice echo signal, and with the far end echo suppressor protecting the near end subscriber against his own echo. With a full echo suppressor, only a single echo suppressor is required in a four-wire transmission circuit, but such suppressors are usually effective only where echo return delay is relatively short. Consequently, split echo suppressors are most commonly employed in long distance telephone communication systems.
Conventionally, split echo suppressors have three distinct operating modes for different transmit and receive channel signal conditions. Considering only the near end echo suppressor, since the suppressors at the near and the far ends are generally identical or at least provide identical suppression functions, the operating mode of the suppressor usually is as follows: (1) when the transmit and receive channel signal levels are representative of neither subscriber speaking, the suppressor is in a quiescent mode and attenuation is removed from both channels; (2) when the transmit channel signal level is representative of the near end subscriber not speaking and the receive channel signal level is representative of the far end subscriber speaking and for a predetermined suppression hangover time thereafter, the echo suppressor operates in a suppression mode in which relatively high suppression or attenuation, typically on the order of 60 dB, is switched into the transmit channel to protect the far end subscriber against an echo signal of his own voice reflected back from the near end over the transmit channel, and (3) when the transmit channel signal level is representative of the near end subscriber speaking and for a predetermined hangover time thereafter, whether or not the far end subscriber is simultaneously speaking, the suppressor operates in a break-in mode and removes the relatively high attenuation from the transmit channel and inserts relatively low attenuation, typically 6 dB, into the receive channel.
In addition to voice signals, such four-wire transmission circuits may also carry single frequency (SF) signaling tones. The SF tones usually comprise a 2600 Hz signal which is transmitted back and forth between distant central switching offices and provides a means of communication or of conveying information between the circuitry at the offices, whereby a transmission path between distant subscribers may be established and controlled. For example, in the most common type of signaling scheme SF tone ordinarily is continuously present on an idle four-wire transmission circuit between distant central switching offices, with each office transmitting an SF tone to the other. Upon a subscriber going off hook or gaining access to the line, the SF tone transmitted by his office to the other is interrupted, thereby indicating to the remote office that a calling subscriber is connected with the line. The SF tone may then be pulsed in accordance with a number dialed by the calling subscriber, so that circuitry at the distant switching office is controlled to establish a connection between the calling subscriber and a particular called subscriber. Similarly, the central offices effect changes in the SF tone to provide, for example, a ringing indication to the calling subscriber. Later, upon the subscribers hanging up at the termination of their conversation, the SF tone is effected by each switching office to indicate to the other that the connection has been interrupted. During the period when a communication link is established between speaking subscribers, SF signaling tones and voice signals may or may not be simultaneously present on the transmission facility.
Since the 2600 Hz SF signaling tones are within the voice frequency band, in transmission circuits which use echo suppressors some means must be provided to prevent SF signaling tones from entering the echo suppressors. Heretofore, an echo suppressor used in a conventional application has been located on the terminal equipment side of SF signaling units, which prevents SF tones from entering the echo suppressor. If such an echo suppressor were inserted at an intermediate point between signaling units, however, the SF signaling tones would be adversely affected or perhaps completely interrupted by the echo suppressor, resulting in loss of switching circuit control functions. Furthermore, the presence of inband signaling tones interferes with the ability of the echo suppressor to reliably detect speech signals, thus interfering with its operation. Thus, the result of a conventional echo suppressor located within an SF signaling link is interference with both signaling and echo control.
One previous technique which enables use of an echo suppressor at an intermediate point between signaling units contemplates disabling the echo suppressor during periods when SF tone is present, so that the tone is not attenuated and the circuit switching functions are not adversely affected. A disadvantage of this technique is that upon the simultaneous presence of speech energy and signaling tones the echo suppressor is disabled and permits echo signals to return to a speaking party. Another disadvantage is the simultaneous appearance of signaling tone energy and another continuous signal, such as dial tone, will result in failure to detect the signaling tone, resulting in failure to properly control the circuit.
Another technique is to position a pair of SF signaling units at the input ports to and output ports from an echo suppressor, and to interconnect the units to block application of SF tone to the echo suppressor and to reinsert the tone into the line subsequent thereto. Such a technique is, to say the least, expensive and cumbersome to implement.
It would therefore be extremely desirable to provide a voice-switched split-type echo suppressor for application at intermediate points in four-wire transmission facilities, which would provide conventional voice-switched echo control without affecting network signaling, even in applications characterized by simultaneous presence of speech or other energy and signaling tones.